This invention relates generally to step-and-repeat alignment and exposure systems utilizing a projection lens of the reduction type for the photometric printing of an image of a first object, such as a reticle, upon a second object, such as photomask or a semiconductive wafer, and, more specifically, to apparatus for use in such systems to achieve precise relative alignments of a reticle and a semiconductive wafer with respect to coordinate axes of motion of a movable stage for holding the semiconductive wafer.
In the semiconductor industry projection lenses of the reduction type are employed both in the fabrication of photomasks and in the processing of semiconductive wafers to form integrated circuits and the like. A high (submicron) resolution photomask is typically fabricated by utilizing a precisely controlled stage to successively position adjacent regions of the photomask with respect to an image (formed by such a projection lens) of a reticle containing a level of microcircuitry that is printed on the photomask at each of those regions. This step-and-repeat printing operation forms an array of adjacent regions of microcircuitry of one level on the photomask in rows and columns parallel to the coordinate axes of motion of the stage. A set of such photomasks, each bearing an array of microcircuitry of a different level, is typically employed in the fabrication of integrated circuits or the like from a semiconductive wafer. In the course of this fabrication, the semiconductive wafer is sequentially aligned with each photomask of the set and the level of microcircuitry printed on the photomask is in turn printed on the semiconductive wafer. However, it is also possible to eliminate the operation of fabricating a set of such photomasks by employing a precisely controlled stage to successively position adjacent regions of the semiconductive wafer with respect to each of the reticles employed in fabricating the set of photomasks so that the level of microcircuitry contained on each of those reticles may be printed directly on the semiconductive wafer at each of those regions during separate step-and-repeat printing operations.
In order to facilitate the precise positioning or alignment of one level of microcircuitry being printed on a semiconductive wafer at each of an array of adjacent regions thereof relative to another level of microcircuitry previously printed or yet to be printed on the semiconductive wafer at each of those same regions, it would be highly desirable to employ a step-and-repeat alignment and exposure system utilizing a projection lens of the reduction type in the optical portion thereof while allowing direct viewing and alignment of a reticle, or an image of the reticle, with respect to the coordinate axes of motion of the stage, and further allowing direct viewing and alignment of microcircuitry previously printed on the semiconductive wafer at each of those regions with respect to the reticle, or image of the reticle. Unfortunately, however, conventional step-and-repeat alignment and exposure systems utilizing projection lenses of the reduction type do not allow such direct viewing and alignment of either the reticle, or an image of the reticle, or the semiconductive wafer.
Accordingly, it is the principal object of this invention to provide an improved step-and-repeat alignment and exposure system incorporating a projection lens of the reduction type in the optical portion thereof while allowing direct viewing and alignment of an image of a first object, such as a reticle, and of a second object, such as a semiconductive wafer.
Another object of this invention is to provide the optical portion of the step-and-repeat alignment and exposure system with a viewing port for observing the image plane of the projection lens.
Another object of this invention is to provide the optical portion of the step-and-repeat alignment and exposure system with masking apparatus for selectively illuminating different portions of the reticle.
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a controlled movable stage having a visible reference mark that is indicative of at least one of coordinate axes of motion of the stage.
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a reticle alignment subsystem for precisely and repeatably aligning an image of each reticle of a set of different reticles with respect to the axes of motion of the controlled movable stage.
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a wafer alignment subsystem for precisely aligning a previously-printed array of adjacent regions of microcircuitry on the semiconductive wafer with respect to the axes of motion of the controlled movable stage.
Still another object of this invention is to provide the step-and-repeat alignment and exposure system with a wafer alignment subsystem for directly aligning the previously-printed regions of microcircuitry on the semiconductive wafer with respect to an image of each reticle of a set of different reticles.
These and other objects, which will become apparant from an inspection of the accompanying drawings and a reading of the associated description, are accomplished according to the illustrated preferred embodiment of this invention by providing a step-and-repeat alignment and exposure system including a main stage controlled for movement to different positions along orthogonal X and Y axes; a chuck mounted on the main stage for supporting a semiconductive wafer thereon; a substage mounted on the main stage for aligning a reference mark on the substage with one of the X and Y axes of motion of the main stage; another stage controlled for aligning an image of a reticle supported thereon with the reference mark; a projection lens of the reduction type mounted between the main stage and the other stage for imaging illuminated portions of the reticle onto portions of the reference mark or the semiconductive wafer, depending on the position to which the main stage is moved; a light source for directing illumination and exposure light along an optical path extending through the reticle; a pair of filters and a compensating lens mounted for selectively controlling the type of light (i.e., whether illumination light or exposure light passing along that optical path to the reticle and for accommodating the projection lens for the type of light selected; a pair of shutters mounted for selectively controlling the passage of light along that optical path to the reticle; a plurality of different mask plates mounted for selectively controlling the portions of the reticle illuminated by the light passing along that optical path when one of the shutters is opened; and a beam splitter mounted between the projection lens and the reticle for providing a viewing port at which an aerial image of the portions of the reference mark or semiconductive wafer illuminated by the projected image of the illuminated portions of the reticle may be viewed.
The step-and-repeat alignment and exposure system also includes a first objective lens unit that may be moved into an operative position for use with an ocular lens unit to permit viewing of the aerial image provided at the viewing port while controlling the main stage is controlled to directly align either the reticle with the reference mark or the semiconductive wafer with the reticle; a pair of prealignment reticles mounted above the main stage and aligned with respect to the reference mark to permit prealignment of the semiconductive wafer with respect to the reference mark (and, hence, the reticle); and a second objective lens unit for imaging this pair of prealignment reticles onto a corresponding pair of alignment marks on the semiconductive wafer when the main stage is moved to position the semiconductive wafer directly beneath the second objective lens unit and for providing aerial images of the pair of alignment marks on the semiconductive wafer illuminated by the projected images of the pair of prealignment reticles. This second objective lens unit may be moved into the operative position (in lieu of the first objective lens unit) for use with the ocular lens unit to permit viewing of these aerial images while the main stage is controlled to align the pair of alignment marks on the semiconductive wafer with the corresponding pair of prealignment reticles. Once the semiconductive wafer has been so aligned, the main stage may be controlled to step an array of adjacent regions of the semiconductive wafer directly beneath the projection lens to permit direct alignment of selected ones of those regions with the reticle while employing the first objective lens unit with the ocular lens unit for viewing an aerial image of an auxiliary alignment mark previously printed alongside each selected region and illuminated by a corresponding alignment mark on the reticle and to further permit printing of a level of microcircuitry contained on the reticle at each of the array of adjacent regions.